Method of forming composite products consisting of ferrous metal and aluminum or aluminum-base alloy



Patented Mar. 13, 1951 UNITED STATES PATENT OFFICE METHOD OF FORMING COMPOSITE PROD- UCTS CONSISTING F FERROUS METAL AND ALUMINUM 0R ALUMINUM-BASE ALLOY Howard L. Grange and Dean K. Hanink, Detroit,

Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application February 12, 1948,

Serial No. 8,008

This invention relates to forming composite articles by casting and bonding aluminum or an aluminum base alloy onto steel or other ferrous The simple In carrying out the invention the steel or other:

ferrous metal part is immersed in a molten or fused salt bath capable of absorbing iron oxide and aluminum oxide. A salt bath of the following composition has proven highly satisfactory.

47% potassium chloride (KCl) 35% sodium chloride (NaCl) 12% cryolite (NaaAlFs) 6% aluminum fluoride (AlFs) This salt bath has a melting point of about 1180 F. Addition of a small amount of lithium chloride to the above composition lowers the melting point thereof. For example, about 20% lithium chloride lowers the melting point to about 1075 F.

The exact composition of the salt bath is not critical and the proportions of potassium chloride, sodium chloride, cryolite and aluminum fluoride may be varied from the above. For example, the potassium chloride may range from about 37 to 57%, the sodium chloride from about 25 to 45%, the cryolite from about 8 to 20% and the aluminum fluoride from about 0.5 to 12%. The bath composition usually preferred is one that will become molten when heated to 1200 F. or somewhat lower. While in the foregoing examples the double salt NasAlFs (cryolite) is given, it should be understood that an equivalent amount of this component may be supplied in the form of the single salts, sodium fluoride and aluminum fluoride. We have found that it is essential to provide an excess of AlFs over that of the cryolite ratio in order to obtain the desired results.

The temperature of the molten or fused salt bath should be maintained above 1250 F. to obtain effective fluxing action. To avoid excessive volatilization and chemical instability of the molten salt it should not be maintained higher than 1600 F. Salt bath temperatures within the range of 1300 to 1450" F. are preferred. The fused salt bath must be activated by aluminum in or in contact with the fused salt in order to provide effective fiuxing action. This may be done by employing an aluminum oraluminum alloy- 8 Claims. (Cl. 22204) coated container forthe fused salt, or aluminum or aluminum alloy can beadded to the salt. The aluminum or aluminum alloy may be added by immersing a bar or sheet of the metal in the fused salt bath. The bar or sheet of aluminum or aluminum alloy soon melts and goes to the bottom of the bath.

The steel or other ferrous metal part may be preheated, if desired, prior to immersion in the salt bath and this permits use of smaller quantities of salt and smaller sized salt bath heating means. Where preheating is employed it is preferred that the ferrous metal be preheated under conditions such that the surface is not oxidized.

For this purpose heating in a non-oxidizing or reducing atmosphere furnace such as one employing hydrogen, Drycolene, etc., may be used. The term Drycolene is the trade-name for a furnace atmosphere gas produced in a charcoal generator utilizing a hydrocarbon gas and air as a gas source. The air and hydrocarbon gas are passed through hot charcoal at 1800 F. and transformed by chemical reaction with the charcoal into an atmosphere consisting of approximately 20% carbon monoxide, less than 2% hydrogen, less than .5% carbon dioxide, and the balance nitrogen. The preheating temperature preferably is on the order of 1200 F. to 1-600 F. Where the ferrous metal is preheated to the temperature of the salt bath in a reducing atmosphere and is free of oxides of iron and other foreign matter, the time of immersion may be as little as one or two seconds if no complicated recesses are present. If complicated recesses are present a longer time may be required to ensure that the salt thoroughly cover or coat the ferrous metal part at those portions thereof to which the aluminum or aluminum base alloy is to be bonded. Where the ferrous metal has oxides of iron or other foreign matter on the surface thereof longer times of immersion will be required in order to obtain clean surfaces. Where the preheating step is not used a longer time of immersion is required. The time should be at least sufficient to heat the ferrous metal part to a temperature of at least about 1250 F. The exact time required will, of course, depend on the mass of the part and the size and thermal efficiency of the furnace and salt containing vessel. Holding the ferrous metal in the fused salt for an extended period of time is not harmful. The fused salt bath serves to clean the steel or other ferrous metalfrom undesirable oxides and to flux the metal preparatory to dipping or immersing in molten aluminum or a molten aluminum-base alloy bath. No special preparation of the surface of the ferrous metal part prior to immersion in the fused salt bath is required since the molten salt bath flux dissolves iron minum alloy gear blank as just described may be precipitation treated at 350 F. for ten hours with a resulting hardness of 94 to 110 Brinell hardness number with a 500 kg. load.

and rust removing procedures may be employed. I

After treatment in the fused salt bath the ferrous metal part is immersed in molten aluminum or molten aluminum base alloy to form a coating of aluminum or aluminum alloy thereon. Best results are obtained when the coating metal bath is maintained at substantially the same temperature as that of the fused salt bath. The time may vary from as little as about one or two seconds up to several minutes, depending somewhat on the degree of complication of recesses, etc. in the part being processed. A relatively short period of immersion'is preferred so as to prevent excessive formation of brittle ironaluminum alloys. Ordinarily, therefore, the ferrous metal being coated is held in the molten aluminum or aluminum alloy bath for ten seconds or less. The fused salt bath and the aluminum or aluminum alloy coating metal may be employed in separate containers, or both the fused salt and fused coating metal may be used in the same container and heated by a single heating means. Where separate containers are used the fused salt is activated as described heretofore. Where a single container is employed for both the salt and coating metal, the aluminum of the coating metal activates the fused salt.

The ferrous metal part with a coating of aluminum or aluminum base alloy thereon is then removed from the coating bath and inserted in located position in a mold, the mold closed, and molten aluminum or aluminum base alloy poured into the mold while the coating is still in a molten or mushy condition. The mold gates preferably are so designed as to direct the incoming molten aluminum or aluminum-base alloy up and around the coated ferrous metal part to thereby carry any oxide films up into the risers of the casting. The coated ferrous metal part should be at or near the temperature of the incoming aluminum or aluminum-alloy to avoid chill and blow effects. After cooling, the ferrous metal and the aluminum or aluminum base alloy are strongly bonded into an integral assembly.

By aluminum-base alloys is meant those alloys which contain about 80% or more of aluminum. One aluminum base alloy that is particularly advantageous, especially as the coating metal, is one composed of 7% tin and the balance aluminum. Other typical aluminum base alloys which may be employed for either or both the coating metal and casting metal are the following: an alloy composed of about 5 to 15% silicon and the balance aluminum, an alloy composed of 4% copper and the balance aluminum, alcoa alloys 142 and 355, and an alloy composed of 5 to 20% zinc and the balance aluminum. These specific examples of typical aluminum base alloys are given merely for purposes of illustration and not of limitation.

In one application of the invention the process has been employed in forming gear blanks consisting of a steel hub to which was bonded an aluminum alloy web and flange portion. In this instance the steel hub was formed of G. M. 1112 steel, the coating alloy consisted of 7% tin and the balance aluminum, while the aluminum casting alloy was alcoa alloy #142. The alu- The invention has application to forming any composite article consisting of steel or ferrous metal to which is bonded a layer of aluminum thicker than can be readily applied by a dipping process.

We claim:

1. The method of forming a composite product of ferrous metal and a metal consisting of at least about aluminum which comprises immersing the ferrous metal in a fused salt bath composed substantially as follows: 3'7 to 57 KCl; 25 to 45 NaCl; 8 to 20% NasAlFt; 0.5 to 12% AlFa; said fused salt being activated by aluminum in contact therewith, said fused salt being at a temperature within the range of about 1250 F. to about 1600 F., said ferrous metal having a temperature while in said fused salt bath, of at least about 1250 F., then immersing the heated ferrous metal in a molten metal coating bath consisting of at least about 80% aluminum, then removing the coated ferrous metal from the coating bath and, before the coating has completely solidified casting molten metal consisting of at least about 80% aluminum into contact with said coated ferrous metal.

2. A method as in claim 1 in which the ferrous metal is preheated under non-oxidizing conditions prior to immersion in the fused salt bath.

3. A method as in claim 1 in which the ferrous metal is immersed in the fused salt bath while the ferrous metal is at room temperature and is heated in said fused salt bath to a temperature of at least 1250 F.

4. A method as in claim 1 in which a small proportion of lithium chloride ranging up to 20% is added to the fused salt bath.

5. A method as in claim 1 in which the fused salt bath is operated at a temperature within the range of 1300-1450 F.

6. A method as in claim 1 in which the ferrousmetal is steel.

7. A method as in claim 1 in which the coating metal is an alloy composed of about 7% tin and the balance aluminum.

8. The method of forming composite articles of steel and a metal consisting of at least about 80% aluminum which comprises immersing the steel in a fused salt bath composed substantially as follows: 47% potassium chloride; 35% sodium chloride, 12% cryolite, and 6% aluminum fluoride, said fused salt bath being activated by aluminum in contact therewith, said fused salt bath being maintained at a temperature Within the range of about 1300" F. to 1450" F., said steel having a temperature of at least about 1250 F. while in said fused salt bath, immersing the heated steel in a molten metal coating bath consisting of at least about 80% aluminum for a time within the range of approximately one to ten seconds, then removing the coated steel from the coating bath, and, before the coating has completely solidified inserting the coated steel in a mold and casting molten metal consisting of at least about 80% aluminum into contact with the coated steel.

HOWARD L. GRANGE. DEAN K. HAN'INK.

{References on following page) REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Gooch Jan. 11, 1898 Monnot Jan. 25, 1910 Dean May 8, 1923 Post Aug. 18, 1925 Pacz Nov. 13, 1928 Deputy June 2, 1931 Johansson Jan. 2, 1934 Number Number 

1. THE METHOD OF FORMING A COMPOSITE PRODUCT OF FERROUS METAL AND A METAL CONSISTING OF AT LEAST ABOUT 80% ALUMINUM WHICH COMPRISES IMMERSING THE FERROUS METAL IN A FUSED SALT BATH COMPOSED SUBSTANTIALLY AS FOLLOWS: 37 TO 57 KCL; 25 TO 45 NACI; 8 TO 20% NA3ALF6; 0.5 TO 12% ALF3; SAID FUSED SALT BEING ACTIVATED BY ALUMINUM IN CONTACT THEREWITH, SAID FUSED SALT BEING AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 1250* F. TO ABOUT 1600* F., SAID FERROUS METAL HAVING A TEMPERATURE WHILE IN SAID FUSED SALT BATH, OF AT LEAST ABOUT 1250* F., THEN IMMERSING THE HEATED FERROUS METAL IN A MOLTEN METAL COATING BATH CONSISTING OF AT LEAST ABOUT 80% ALUMINUM, THEN REMOVING THE COATED FERROUS METAL FROM THE COATING BATH AND, BEFORE THE COATING HAS COMPLETELY SOLIDIFIED CASTING MOLTEN METAL CONSISTING OF AT LEAST ABOUT 80% ALUMINUM INTO CONTACT WITH SAID COATED FERROUS METAL. 